Recent clinical studies utilizing repeat arteriography indicate that progression of atherosclerotic lesions rarely can be halted in symptomatic individuals. The question has therefore been raised whether development and progression of lesions should not be prevented by risk factor intervention early in life. To decide whether or not to intervene in the life of young, unsymptomatic individuals, and to decide on the means of the intervention, data must be obtained on the nature of lesions in the arteries of young individuals in this country at this time. Since the lesions are probably not of a single origin, the relative incidence, and themechanism and rate of progression of each type, must be established. To contribute this information is the aim of this study. We will investigate the coronary arteries and aortas of individuals of both sexes, black and white, aged from fullterm birth to 39 years, obtained post-mortem at the Morgue of the Coroner of New Orleans (Parish of Orleans) and at Charity Hospital of New Orleans. We will specifically study the following points: changes in the nature and quantity of the cellular and extracellular components of fatty streaks that might indicate progressive transition into the potentially obstructive fibrous plaques, and the reasons for it; the presence of fibrin, platelets and organizing thrombotic encrustations on or within intima, fatty streaks, or fatty streak-derived plaques, pointing to plaque generation or to accelerated plaque progression through this mechanism; the influence on fibrous plaque development of individual and regional arterial deviations, and changes caused by age; the relative frequency of these mechanisms. We will use light microscopy of 1-Mum. plastic-embedded sections, and electron microscopy to identify and quantitate cellular, extracellular matrix, and circulation-derived components of intima, media, adventitia and lesions. Coronary arteries will be pressure-perfusion fixed at 100 mm. Hg. To identify and quantitate cell types, fibrin, and platelets, we will apply specific immunological markers. To characterize and quantitate the functional status of cells we will use enzyme cytochemistry, and to determine changes in initimal lipid we will use chemical lipid analysis.